It is known, for instance from German patent document No. 2,819,193 filed May 2, 1978 by F. Voigt et al to control the motor driving the vacuum blower of a vacuum cleaner by means of a standard diac-triac voltage controller whose setting potentiometer is in turn controlled in accordance with pressure in the cleaner's hose. Thus pressure at the intake port of the system is detected and when it increases the blower speed is decreased and when it decreases the pressure is increased to keep it fairly uniform. In this manner, for instance, when a nozzle connected to the vacuum cleaner moves from a smooth floor where it fits relatively well so that high suction can be developed with a low motor speed to a shag rug with considerable leakage, motor speed in increased to compensate for the increased leakage.
As applied to industrial systems, for instance in a textile mill where a common intake duct may be connected to a plurality of intake ports that can each be fitted with a hose and nozzle for aspirating particles or fibers at different locations in the plant, such a system is inadequate. The large typically three-phase motor that drives such an industrial system cannot be shifted so readily to match its speed to vacuum requirements, and in fact the inertia of the system makes control fairly difficult. In particular it is possible when the system is started to overload and stall the drive motor as it works against a substantial suction head, while this head can reverse-drive the motor when the system is shut down to cause flow in the system to reverse momentarily, a phenomenon called puffback.